Hydroisomerization is a reaction whereby normal paraffins are converted to isoparaffins and is considered as an important process in petroleum refining.
Since normal paraffins have high pour point, cloud point, etc., which are important factors of the flow characteristics of a liquid, long-carbon-chain hydrocarbon materials containing a lot of normal paraffins, such as diesel or lube base oil, are inadequate for use as fuel because they are easily solidified at low temperatures in the winter season. As a consequence, additives have to be mixed in diesel, lube base oil, etc. in order to improve the flow characteristics. But, if the normal paraffins are converted to isoparaffins, they can be immediately used as fuel without any additive because the flow characteristics of liquid such as pour point and cloud point are improved.
In general, the isoparaffins having improved flow characteristics of liquid are produced from hydroisomerization using a catalyst having a metal part and an acid part at the same time. As the metal part, a precious metal such as platinum (Pt) or palladium (Pd) supported on a support at 0.3-1.0 wt % is used in general. And, as the acid part, a zeolite having a 1-dimensional channel structure and having pores, each of which consists of 10 atoms is frequently used because it is known to prevent hydrocracking and give a high yield of mono-branched isomers.
Micropores (pore diameter <2 nm) present in common zeolites are disadvantageous in catalytic reactions in that the molecular diffusion of reactants (hydrocarbon materials containing normal paraffins) and products (hydrocarbon materials containing isoparaffins) is limited. In particular, for hydroisomerization, limited diffusion of the products, isoparaffins, causes hydrocracking inside the micropores and thus decreases the isoparaffin yield. To solve this problem, studies are under way on the synthesis of a zeolite having a large outer surface area to enhance molecular diffusion in the pores of the zeolite.
Zeolites have a wide variety of structures, including the AFI, AEL and CHA structures having pores consisting of 12, 10 and 8 atoms, respectively. The three-letter codes such as AFI, AEL, CHA, etc. designate the structure types defined by the International Zeolite Association and the pore structures and characteristics can be found in the published literature (Chem. Review, 99, 63). There are also zeolites containing metals other than aluminum, phosphorus and oxygen. For example, the AIPO (aluminophosphate)-based zeolites of the AEL structure containing Ti, V, Cr, Mn, Fe and Co metals are designated as TAPO, VAPO, CrAPO, MnAPO, FAPO and CoAPO, respectively. Also, numbers are used to designate different structures. For example, CoAPO-5, CoAPO-11 and CoAPO-34 designate the zeolites consisting of Co, Al, P and O, with the AFI, AEL and CHA structures, respectively.
A method of preparing the SAPO-11 zeolite by using an organosilane surfactant as a silicon precursor to improve material diffusion in the SAPO-11 zeolite was reported (M. Choi et al., Chem. Commun. 42; 4380, 2006). The SAPO-11 zeolite synthesized using the organosilane surfactant forms mesopores (pore diameter: 2-50 nm) during calcining as the structure directing agent and organic matter present in the organosilane are removed. As a result, material diffusion is improved.
Also, a method of preparing the SAPO-11 zeolite by using a surfactant (decyltrimethylammonium bromide (C10H21(CH3)3NBr), dodecyltrimethylammonium bromide (C12H25(CH3)3NBr), tetradecyltrimethylammonium bromide (C14H29(CH3)3NBr) or hexadecyltrimethylammonium bromide (C16H33(CH3)3NBr)) was disclosed (Lin Guo et al., J. Catal. 294; 161, 2012). When hydroisomerization of n-octane was conducted using a catalyst for hydroisomerization of normal paraffins prepared using the SAPO-11 zeolite synthesized by using the surfactant, the yield of multi-branched isomers including di-branched isoparaffins was increased by about 10% as compared to the conventional SAPO-11 zeolite.
However, there are problems that a catalyst for hydroisomerization of normal paraffins prepared using the SAPO-11 zeolite with increased outer surface area synthesized by using the organosilane surfactant exhibits low isoparaffin yield and the catalyst for hydroisomerization of normal paraffins prepared using the SAPO-11 zeolite synthesized by using the surfactant such as decyltrimethylammonium bromide (C10H21(CH3)3NBr), etc. is costly.
Accordingly, in order to solve these problems, a zeolite which is more economical and exhibits high isoparaffin yield and hydroisomerization reactivity and synthesis of a catalyst for hydroisomerization of normal paraffins using the same are necessary.